Previously we observed that wild type human rotavirus (strain D contained in a suspension of feces) induced diarrheal illness in 5 of 18 adult volunteers. During this study serologic correlates of resistance were identified. Following adaptation of the Wa strain of human rotavirus to tissue culture we used this information to identify susceptible individuals so that we could investigate virulence of the Wa mutant for adult volunteers. Six of 10 volunteers with a low level of serum antibody were infected but did not become ill. This suggested that the Wa rotavirus mutant was attenuated compared to the wild type D rotavirus previously studied in adult volunteers. Three of the Wa volunteers developed a mild transaminase elevation 10 days post challenge; this response was transient and did not appear to be related to rotavirus infection. These 3 individuals again developed a mild transaminase elevation several weeks later. Chimpanzees and rhesus monkeys also developed a mild transient rise in transaminase 8 to 11 days after inoculation with the tissue culture grown Wa inoculum. The cause of the transaminase elevations is not clear at this time. Currently, 4 other rotaviruses are being evaluated for their capacity to induce transaminase elevation in primates. Those strains that do not induce this response will be considered for subsequent study in volunteers.